Rare-earth sintered magnets currently used extensively in various applications include rare-earth-cobalt based magnets and rare-earth-iron-boron based magnets. Among other things, the rare-earth-iron-boron based magnets (which will be referred to herein as “R-T-(M)-B based magnets”, where R is one of the rare-earth elements including Y, T is either Fe alone or a mixture of Fe, Co and/or Ni, M is an additive element (e.g., at least one of Al, Ti, Cu, V, Cr, Ni, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta and W) and B is either boron alone or a mixture of boron and carbon) are used more and more often in various electronic appliances. This is because an R-T-(M)-B based magnet exhibits a higher maximum energy product than any of various other types of magnets and yet is relatively inexpensive.
As the applications of those rare-earth sintered magnets expand, it becomes increasingly necessary to produce magnets in various shapes. For example, to produce a high-performance motor, a number of strong anisotropic magnets with a curved surface are required. To produce such an anisotropic magnet, a powder compact needs to be compacted into a desired shape by pressing a magnetic powder that has been aligned under a magnetic field. A high-performance rotating machine such as a motor uses a plurality of thin-plate magnets with a C- or arched cross section. The performance of such a rotating machine cannot be improved sufficiently just by increasing the magnetic force of the magnets. In addition, the resultant magnet shape and magnetic field distribution in the vicinity of the magnet surface also have to be just as designed.
In the prior art, the pair of punches of a press machine has curved press surfaces, thereby obtaining a powder compact with desired curved surfaces. The conventional punches may be made of a cemented carbide (e.g., a WC—Ni based alloy) and the press surfaces thereof may be mirror polished.
However, the present inventors discovered and confirmed via experiments that if the press surfaces were mirror-polished curved surfaces while a magnetic powder was uniaxially pressed under an aligning magnetic field, then the orientation of the magnetic powder was disturbed and the resultant magnet performance was not so good. This problem is quite noticeable particularly when the pressing direction is substantially the same as the direction of the aligning magnetic field.
If permanent magnets are made of such a compact with the disturbed orientation and used to produce a motor, then a non-negligible degree of cogging will be created in the torque of the motor. The “torque cogging” is a torque variation resulting from a variation in the magnetic resistance of the magnetic circuit of a motor with the rotational position of a rotor. The magnitude of this torque variation is usually small. However, if the cogging torque phenomenon occurs in a power steering motor, for example, then that variation could be quite sensible to some drivers. This torque cogging becomes even more perceivable when there is such disturbed orientation in the convex portion of each magnet (i.e., a portion of the motor facing a coil).
The above-mentioned problem that a magnetic powder, which has been aligned under a magnetic field and is now being pressed uniaxially, can have disturbed orientation arises not only when the press surfaces are curved but also when one of the two press surface has a region that is tilted with respect to the pressing direction. This phenomenon occurs during the manufacturing process of magnets in any of various shapes.
Also, a slicing technique is often used as a method for filling a cavity with a magnetic powder. For example, as disclosed in Japanese Laid-Open Publication No. 2000-248301, a feeder box (or a feeder cup) is slid over a cavity, a powder in the feeder box is loaded into the cavity by utilizing the weight of the powder itself, and the upper portion of the loaded magnetic powder is pressed downward by some pressing means such as an agitator (also called a “shaker”) provided within the feeder box 13.
However, the surface of a magnetic powder that has been loaded by such a slicing technique is not always parallel to the surface of a die (i.e., the bottom of the cavity) but may be either tilted in the direction in which the agitator (or the feeder box) moves or even winding. In that case, even if the magnetic powder is pressed between two mutually parallel press surfaces, at least a portion of the upper press surface (i.e., the surface of the upper punch) contacts with the surface of the magnetic powder obliquely. If the magnetic powder that has been aligned under the magnetic field is pressed in such a state, then the magnetic powder particles in the vicinity of the press surface will also have disturbed orientation due to the movement of the compressed magnetic powder just as described above.